"we were recording dialog voice over"
I assume separate mics? Issues with mic cable? - had these quite a few times (humming cables). Last year I had an older (80s) mic with intermittent hum / grounding issues. If I held mic in hand - no humming. As soon as it went on stand -hum. I re-soldered internal connector and problem was gone. In any case, if it's mic related, I would try swapping cables / try another mic on same humming channel.